What would cause the engine to intermittently die while at idle?

Never had this issue with the truck or any other vehicle before. While at idle in traffic, intermittently the engine will start to pulse between roughly 500 rpm's and nearly none. The pulsing will start off minimal but grow to the point that it will finally die. I can throw it into park and restart it no problem, but the pulsing continues. Otherwise I have no problems with the engine while driving. I have noticed that it only seems to do this when in gear, not when in park.

Make certain there are no possible air leaks in the 'big air tube' from the air cleaner box to the intake .. Look closely for any cracks, loose clamps or missing attachments about the air tube... Also, engine vacuum leaks can cause rolling idle.

Make certain there are no possible air leaks in the 'big air tube' from the air cleaner box to the intake .. Look closely for any cracks, loose clamps or missing attachments about the air tube... Also, engine vacuum leaks can cause rolling idle.